This week, the Therapeutic Goods Administration (TGA) put out an alert saying children under the age of six should not be given promethazine hydrochloride.

The medication, commonly known by the brand name Phenergan, is used to treat allergies and hayfever, and for short-term sedation. 

Here's what parents need to know about the new rules. 

What has the TGA said about the drug?

They say it shouldn't be used in children under the age of six. 

"These products can still be used in children six years and older for their approved indications," a TGA spokesperson said. 

The advice for children under six is, at the moment, only for the oral form of the drug. 

"At this stage, the updated advice does not apply to the single intravenous form of promethazine hydrochloride on the Australian market, noting that this product is only available with a prescription from a doctor."

Why can't kids under six take it?

The TGA says there are "risks of psychiatric and central nervous system side effects" that include:

• hyperactivity
• aggression 
• hallucination

"When high doses are given, these children may also experience difficulties in learning and understanding, including reversible cognitive deficit and intellectual disability," the TGA said in its safety alert. 

What do we know about the side effects?

We spoke to Nitin Kapur, a paediatrician and director of paediatric education at the Queensland Children's Hospital, about the psychiatric and central nervous system side effects. 

"These adverse effects have been well known though rare," Dr Kapur said. 

"[I] don't think the effects are long term, so if parents haven't seen these yet, they will not see it if they stop Phenergan now."

He said these side effects are usually only present while the child is taking the medication. 

We also asked him about the possibility of reversible cognitive defect and intellectual disability.

"Reversible means side effects will improve once medicine stopped," he said. 

How high is the risk?

The ABC asked the TGA about how high the risk of side effects was for children under the age of six using the medication.  

A spokesperson for the TGA didn't give a specific figure, but said the recommendation was based on a review by Sanofi-Aventis Healthcare, which is the pharmaceutical company that markets Phenergan.

"This data demonstrated sufficient evidence that promethazine hydrochloride could potentially cause psychiatric and central nervous system side effects in [the two- to five-year-old] age group," the spokesperson said. 

"The TGA applies scientific and clinical expertise to its decision-making to ensure that, based on currently available data, the overall benefits of a product outweigh the risks in the intended population, when used for the approved indications."